1. Field of the Invention
The present invention relates to a liquid developer, and more specifically to a positively-charged liquid developer.
2. Description of the Prior Art
As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known a liquid developer which is obtained by dispersing toner particles composed of a toner material containing a binder resin and a coloring agent such as pigments into a carrier liquid having electric insulation properties, that is, an insulation liquid. Generally, a polyester resin is used for such a binder resin for toner particles contained in a liquid developer. Polyester resins have high transparency, and thus when such resins are used as a binder resin, there are advantages in that obtained images can provide excellent color development and excellent fixing characteristics.
In the meantime, such liquid developers include two types, one of which is a negatively-charged liquid developer and the other of which is a positively-charged liquid developer. However, when such a negatively-charged is used, there is a problem in that ozone is generated in an image forming apparatus, and generation of such ozone not only causes an environmental problem but also gives any adverse effects to peripheral components provided in the image forming apparatus.
On the other hand, in the case of a positively-charged liquid developer, images can be formed with less generation of corona products such as ozone and the like. Therefore, in recent years, researches and developments are conducted for methods for forming images using positively-charged liquid developers (one example of such a liquid developer is disclosed in JP-A No. 2002-214849).
However, generally, polyester resins are negatively-charged type resins. Therefore, it is difficult to use a polyester resin to positively-charged toner particles (liquid developer). Further, there may be an approach that toner particles formed using a binder resin made of a polyester resin are allowed to be positively-charged by adding a charge control agent to the binder resin. However, according to this approach, it is difficult to obtain a sufficient amount of positive toner charging.